1. Field of the Invention
The present invention relates to a magnetic recording medium. More particularly, the invention relates to an improvement of a magnetic recording medium comprising a nonmagnetic support and a magnetic recording layer.
2. Description of Prior Arts
A magnetic recording medium such as an audio-tape, a video-tape, or a recording medium employed in a computer system, basically comprises a nonmagnetic support and a magnetic recording layer provided on the support. The magnetic recording layer comprises a ferromagnetic powder such as a needle crystalline powder of .gamma.-Fe.sub.2 O.sub.3, Co-containing ferromagnetic iron oxide, or CrO.sub.2 dispersed in a binder. Recently, a demand for high density recording system has increased, and accordingly studies for a magnetic recording medium replacing a ferromagnetic metal powder for the conventional oxide-type ferromagnetic powder have been made.
A ferromagnetic metal powder has been employed as ferromagnetic material of a recording medium for computer system. The ferromagnetic metal powder is high in coercive force (Hc) and residual flux density (Br), and moreover is stable in a wide temperature range. For these reasons, the ferromagnetic metal powder has been paid attention for the use in such magnetic recording media as a video-tape or an audio-tape. The ferromagnetic metal powder is known to afford recording of higher density, as the powder is minimized.
While the ferromagnetic metal powder has excellent characteristics, there is a drawback in that the ferromagnetic metal powder is low in hardness. In more detail, a magnetic recording layer using the metal powder shows poor physical resistance in the running operation, that is, the recording layer is easily scratched or the metal powder is easily dropped off from the layer in the course of running in contact with a magnetic head. These troubles cause the so-called drop-outs. Further, the dropped metal powder is apt to adhere to the surface of the magnetic head to cause clogging on the head. Furthermore, if the recording layer is poor in the physical resistance, the recording layer shows only short still life in the still mode in which a still video image is continuously reproduced.
It is known that the physical resistance of a magnetic recording layer is improved by incorporating a globular abrasive (i.e., hard particles) such as corundum, silicon carbide or chromium oxide in the recording layer. However, the effect of incorporation of an abrasive is shown only where the abrasive is incorporated in a large amount. A magnetic recording layer containing a large amount of an abrasive likely causes extreme abrasion of a magnetic head placed in contact with the running magnetic recording layer. Further, a large amount of the incorporated abrasive is unfavorable to the electromagnetic conversion characteristics of the recoriding medium. In more detail, the increase of the amount of the abrasive necessarily brings about decrease of the amount of the ferromagnetic powder incorporated in the recording layer. The decrease of the amount of ferromagnetic powder inevitably results in deterioration of the electromagnetic conversion characteristics.
For the above-described reasons, the conventional measure for improving the physical resistance of the magnetic recording layer is not thought to be satisfactory, in view of the resulting characteristics of the magnetic recording medium as well as unfavorable effect given to the magnetic head.
Further, it has been now discovered that the globular abrasive gives adverse influence to the orientation of the ferromagnetic powder in the magnetic recording layer. It is thought that the conventional ferromagnetic powder in the form of needle crystals is adversely effected by the adjacently existed globular abrasive in the orientation treatment. Thus, the ferromagnetic powder is likely dispersed non-uniformly in the recording layer.
Accordingly, improvement of the physical resistance of the magnetic recording layer is desired in a recording medium particularly using a ferromagnetic metal powder. Such improvement of the physical resistance of the magnetic recording layer is also desired in the conventional recording medium using such a ferromagnetic powder as an oxide-type powder of .gamma.-Fe.sub.2 O.sub.3, Co-containing ferromagnetic ferrite, or CrO.sub.2.